The steam generators of pressurized water nuclear reactors generally include a cylindrical jacket of great height within which a bundle composed of a large number of tubes of small diameter bent into a U is arranged vertically.
Each of the tubes of the bundle is connected through one of its two ends to a tube plate arranged horizontally in the vicinity of the lower part of the steam generator, above the domed lower base of this generator.
The lower part of the steam generator situated beneath the tube plate is partitioned into two substantially identical compartments, of which one communicates with one of the ends of all the tubes of the bundle, and the other communicates with the other ends of these tubes. The water under pressure bringing the heat from the core of the nuclear reactor to the steam generator for the vaporization of the supply water is introduced into one of the compartments, flows in the tubes of the bundle, and comes back into the second compartment; this water under pressure which has yielded a portion of its heat for the vaporization of the feed water is returned to the core of the nuclear reactor.
The group of arms of the U-tubes communicating with the pressurized water inlet compartment constitutes the hot branch of the bundle of tubes, while the group of arms of U-tubes communicating with the other compartment constitutes the cold branch. Between the hot branch and the cold branch of the bundle there usually exists a free space of a certain width.
The supply water to be vaporized is introduced into the upper part of the steam generator and comes into contact with the tube bundle traversed by the pressurized water at high temperature, which causes the progressive vaporization of the feed water.
The steam produced is led off at the upper part of the steam generator and sent to the turbine associated with the electrical generator.
The presence of feed water in the steam generator causes, during the operation of the latter for long periods, the formation of deposits of sludge which accumulate on the tube plate around the U-tubes whose ends are fixed by rolling expansion and welding to this plate. This sludge is composed principally of iron oxide and in particular of magnetite. These deposits are the cause of corrosion phenomena of the tubes at the level of the tube plate, and it is hence necessary to remove them to avoid more or less rapid deterioration of the steam generator.
It has been proposed, for example, to arrange drainage tapping ramps at the level of the tube plate of the steam generator in the space existing between the hot branch and the cold branch of the bundle.
Such ramps permit a part of the sludge which can accumulate on the tube plate to be removed during the operation of the steam generator, but a portion of the impurities transported by the feed water is nonetheless deposited at places where the tubes are joined to the tube plate.
Devices have therefore been proposed enabling the tube plate to be unclogged during shut-offs reserved for the maintenance of the steam generator.
For example, it has been proposed to introduce through a hole provided in the enclosure of the steam generator, immediately above the tube plate, a lance enabling water jets to be sent under very high pressure tangentially to the tube plates. This lance is moved tangentially until it reaches the center of the tube plate where it has passed slightly beyond the center of this plate.
In a second phase, the lance is introduced through another opening placed in a diametrically opposite position from that of the first opening to clean the second half of the tube plate.
During the whole of this unclogging operation a flow of water is provided on the tube plate, which permits the draining of the loosened sludge, through purging orifices.
This device however, has drawbacks since it is not usable during operation, and thus corrosion-inducing deposits remain within the junction zone between the tubes and the tube plate for a length of time which is not negligeable, namely, between successive maintenance operations.
On the other hand, the time required for the cleaning of the tube plate is very long since it must carry out successively, in moving the lance from the inside of the steam generator, the cleaning of the two halves of the tube plate.
There exist various types of devices enabling cleaning liquid to be collected and the sludge to be detached from the tube plate. For example, there is known a ramp of annular shape arranged at the periphery of the tube plate collecting the sludge during the operations of cleaning the tube plate.
These devices do not however, give complete satisfaction for the complete removal of the sludge freed by the water jets, and it is difficult to assure thorough cleaning of the tube plate.
In other respects, steam generators have already been manufactured including a hollow central support column for the tube plate arranged between the lower surface of the latter and the domed lower end of this generator.
It has never, however been conceived to use this hollow central column for the removal of disengaged sludge from the tube plate of the steam generator.